This invention relates to a method and apparatus for manufacturing superconducting magnet coils for use in magnetic resonance imaging (herein after called "MRI"), and more particularly, to an insulated superconductor for use in superconducting magnet coils.
As is well known, a magnet can be made superconducting by placing superconducting magnet coils in an extremely cold environment, such as by enclosing it in a cryostat, or pressure vessel, containing liquid helium or other cryogen. The extreme cold reduces the resistance in the magnet coils to negligible levels, such that when a power source is initially connected to the coil (for a period, for example, of 10 minutes) to introduce a current flow through the coils, the current will continue to flow through the coils due to the negligible resistance, even after power is removed, thereby maintaining a magnetic field. Superconducting magnets find wide application, for example, in the field of medical magnetic resonance imaging.
The superconducting magnetic field is generated by current flow through superconducting magnet coils, typically having a diameter on the order of 45 to 75 inches and including six or more main magnet coils, each having as many as 1500 turns. The many turns and layers of turns must be insulated from each other and the insulation integrity must be maintained during superconducting operation, including the ramping up of the coils to the operating current, and the possible sudden quenching, or discontinuance, of superconducting operation. Any shorting of magnet coil turns can produce heat which can lead to very serious inadvertent quenching of magnet operation which leads to sudden escape of helium and the need to replenish the liquid helium and again ramp the MRI magnet up to operating current entailing significant equipment down time and expense. Such shorting could also damage the superconducting wire, rendering the magnet inoperable. Because of the extremely high current levels involved in the high strength magnetic fields, the superconducting magnet coils must undergo extreme and difficult operating environment and forces, including significant thermal, magnetic, electrical and mechanical forces generated during such operations. As a result, the cost of producing superconducting magnet coils and magnets is relatively high, frequently costing into the hundreds of thousands of dollars for a single MRI magnet assembly. The use of "barber pole" spaced windings with insulating separators of materials such as Nomex between coil layers adds significantly to the material and labor required to fabricate superconducting magnet coils. In addition, problems in winding magnet coils due to friction between pockets in the coil support, or cartridge and the insulated superconductor can result in defects and failures in operation.
As a result, it is extremely important that superconducting magnet coils be capable of reliable operation without failures, and yet must be manufactured as inexpensively as possible in order to reduce the high cost of MRI3 s, to make such equipment more universally available.